At 92 years old, Alfred Donovan is an unlikely online campaigner. But he and his son John, 62, have been a painful thorn in the side of Royal Dutch Shell for more than a decade. The pair run one of the oldest and most effective "gripe sites", and the oil giant's army of well-paid lawyers do not know how to neutralise them.

The number of so-called "gripe sites", which exist to criticise, mock, and generally annoy companies, people, and institutions, has exploded in recent years, and the trend is set to continue.

Take this month's campaign against the super-injunction obtained by the lawyers Carter Ruck on behalf of Trafigura. Thousands of Twitter users, empowered and astonished at the campaign's success, are expected to look afresh at how the internet can be used to fight against big business.

"The anti-Trafigura campaign really brought home – even to someone like me – the power of the internet and new media," says John Donovan, a former marketing entrepreneur. "Once, you could never hope to take on companies that had loads of money and lawyers. Now there is an alternative to legal action. You can make a big impact with very little cost."

Last week the Donovans were leafleting outside Shell's London HQ to advertise their website, www.royaldutchshellplc.com. But they hardly need the publicity – the site had more than 2m hits last month – and leafleting was just another way of goading a company they have been at war with since the early 1990s.

The site is so successful that Kremlin officials and US investigators have used it. Journalists, knowing that the site regularly receives juicy leaks from Shell employees, search it for stories. Since setting up his first anti-Shell site in 1995, Donovan estimates he has published about 24,000 articles about the company.

One early and successful gripe site was www.mcspotlight.org, founded after the celebrated McLibel trial involving McDonalds in 1997. Another site, www.ihatedell.net, carved a niche as a forum for critics of the Dell computer company. Dell's answer was to engage with its critics rather than use legal muscle to close them down. In contrast the investment bank Goldman Sachs failed in a legal bid against www.goldmansachs666.com.

Katy Howell, the director of Immediate Future, which specialises in social media, believes Dell made a textbook response to its gripe site. "Dell spoke to its critics and responded to their concerns. They turned a negative into a positive," she says.

The Donovans' campaign was prompted by a grievance over claims that Shell stole intellectual property from their marketing company. The legal bills from four court cases in the 1990s almost crippled the two men. Shell fully investigated the Donovans' claims, and in 1999 agreed a "peace deal" under which the pair got an undisclosed sum. However, the payment was far less than the £1m they wanted. The Donovans claim Shell then breached the agreement by talking publicly about the case. Shell denies breaching any part of the agreement with the Donovans.

Since then, Shell is thought to have contacted the Donovans at least once, using a middle man, to resolve the dispute. John Donovan will not comment on this but shows no sign of agreeing to mediation.

Four years ago Shell was embroiled in a bitter dispute with Russia's environmental regulator over drilling for gas at Sakhalin Island. It was eventually forced to relinquish its majority stake in the project, costing Shell billions in lost revenue. Later, the regulator, Oleg Mitvol, publicly acknowledged the Donovans' help in getting information about alleged claims of environmental abuses by Shell. The company has denied breaking any environmental regulations.

Earlier this year the site disclosed plans for thousands of Shell job losses. And now, Donovan says, he is helping US investigators looking into the award of oilfield drilling licenses, providing them with information leaked to his website.

The site has broadened its coverage to include other stories about the oil and gas industry. "I knew when I started the site that if it was static – just with the same story – people would visit us once and never again," says Donovan. "So I brought in a news element, mixing negative but also positive stories about Shell."

But "kicking" Shell is still the site's raison d'etre, and Donovan has no intention of easing up. "My father is 92. So if I live that long there's still plenty of years to pursue my little hobby."

Shell says of the Donovans: "We disagree fundamentally with much of the information and basis on which they make their allegations."

The Donovans live in Essex but the website is hosted in Dallas, Texas, and is incorporated in America as a non-profit operation. US laws offer better protection against closure attempts. Shell tried to regain the website name, calling the Donovans cybersquatters, but in 2005 the World Intellectual Property Organisation dismissed the application.

Would Donovan stop if Shell waved a large cheque? "It's gone beyond money," he says, but he has no doubt that Shell's lawyers are watching closely, waiting for a slip-up that would give the company a chance to go on the attack.